1. Field of the Invention
This invention relates to a fixing device which is used for an image forming device such as copying machine, printer, etc. which fixes unfixed images on a supporting material.
2. Related Background Art
As the fixing device for fixing unfixed images, there has been widely used a hot roller fixing device which uses a heating roller with a heater built therein and a back-up roller in pressure contact with the heating roller.
Such heating roller is of the type using a rubber layer as the release layer and of the type using a release layer, but it is advantageous to use a rubber layer for obtaining high fixability. As such rubber, silicone rubber is generally employed for elastic characteristics and mold releasability.
Also, for prevention of off-set, silicone oil is coated on the surface rubber layer. However, fixing by such an image fixing device has been found to generate the following inconveniences.
That is, silicone oil coated as the release agent on the silicone rubber layer of the fixing roller is transferred onto toner and transfer paper during fixing, thereby effecting fixing of the toner image T onto the transfer paper, but the silicone oil remains on the fixing roller in a considerable amount and will penetrate the silicone rubber with time.
The penetration of silicone oil into the silicone rubber layer is further promoted when the fixing roller is heated, rotated, and when the silicone rubber layer receives stress from the pressurizing roller by rotation.
The silicone oil penetrated into the silicone rubber layer promotes pyrolysis of HTV silicone rubber constituting the rubber layer by heat, pressure. If this state continues for a long time, the silicone rubber layer will undergo lowering in hardness as the result of progress of pyrolysis, and further the rubber layer is swelled with the silicone oil, until finally chemical bonding of the HTV silicone rubber is cleaved to effect scission of the rubber. As the result, there occurs the phenomenon that the silicone rubber layer is peeled off from the fixing roller.
Such phenomenon is liable to occur as the temperature is higher, particularly at the silicone rubber layer near the core metal made of aluminum. This may be considered because decomposition of the rubber proceeds greatly as the temperature is higher. For example, even if the thermal conductivity of the HTV rubber may be made as high as 1.0.times.10.sup.-3 cal/cm.multidot.sec.multidot.deg, the temperature in the vicinity of the core metal which will vary depending on such conditions as outer air temperature, heat content of the heater, rotation of the roller, etc., may sometimes become 200.degree. C. when the surface temperature is 170.degree. C.
This is not limited to HTV silicone rubber, but the same phenomenon also occurs with RTV (room temperature vulcanizabIe type) rubber such as dimethylsilicone rubber or methylvinyl silicone rubber or LTV (low temperature vulcanizable type) silicone rubber.
The phenomenon as described above depends on the use time accompanied with heating of the image fixing device, but the time before generation does not differ greatly, although it may vary to some extent depending on the kind of the rubber constituting the silicone rubber layer, the viscosity of the silicone oil, its coated amount, or the heating time, pressurization conditions, rotational conditions of the fixing roller.
For prevention of silicone rubber with silicone oil, for example, Japanese Patent Publication No. 54-26373 proposes a combination of a silicone rubber and a silicone oil so that the rubber layer will not will swell only a or swelled little by selecting the kind of the silicone rubber constituting the rubber layer of the fixing roller, etc. and the kind of the silicone oil coated.
According to the above proposal, as the silicone oil, those of chloro type, fatty acid modified type, nitrile type or fluoro type are employed, and as the silicone rubber, those of methyl type, methylvinyl type or phenyl type are used. Alternatively, as the silicone oil, those of methyl type, phenyl type, methylhydrogen type, methylphenyl type, dimethylphenyl type, chloro type, fatty acid modified type, nitrile type or fluoro type are used, and as the silicone rubber, those of nitrile type or fluoro type are employed (however, when those of nitrile type or fluoro type are used as the silicone oil, silicone rubbers of the same type are excluded).
According to such combination of silicone oil and silicone rubber as mentioned above, it is stated that swelling of the silicone rubber constituting the rubber layer of the fixing roller, etc. with silicone oil can be prevented.
The following Table 1 is an extraction of a part of the combinations from the above proposal. According to this, when one of methyl type, methylvinyl type, and phenyl type is used as the silicone rubber, and one of methyl type, phenyl type is used as the silicone oil, swelling occurs in the rubber, and therefore such combination is stated to be undesirable.
TABLE 1 ______________________________________ Silicone oil Rubber: Methyl type Phenyl type ______________________________________ Methyl type x x Methylvinyl type x x Phenyl type x x ______________________________________ x: indicating that rubber is swelled
As observed from the aspect of mold releasability, methyl type or methylvinyl type or phenyl type silicone rubber is excellent as the rubber material.
In Japanese Patent Publication No. 54-26373, for methyl type, methylvinyl type silicone rubber, silicone oils of chloro type, fatty acid modified type, nitrile type or fluoro type are mentioned as silicone oils which are not swelled with oils, but these materials have lower heat resistance or can be prepared with difficulty to be expensive, and hence not suitable as the coating oil for heating fixing rollers.
As a coating for the heating fixing rollers, silicone oils of methyl type or phenyl type are suitable from the point of view of heat resistance and bulk productivity, but they are combinations which are swelled with methyl type, methylvinyl type, phenyl type silicone rubbers as described above.